Urban Nature Atlas

Green Roofs for Sustainable Water Management

Tetti Verdi dell'Università di Bologna

This project is the first green roof in the city of Bologna, and it has been carried out by the University of Bologna in collaboration with the Columbia University of New York with the aim to prove more evidence on green roof stormwater performance. For the Engineering School of Bologna University the existing roofs had a load capacity that was able to host only an extensive roof, without the costly need of being reinforced. In the extensive roof type the vegetation is usually very drought resistant and plants can adapt to difficult environmental conditions. Therefore sedum was chosen (1).

Main beneficiaries

Researchers/University

Project objectives

The goals of the intervention consisted in analysing the potential impact green roofs have on containing water runoff and reducing the heat island phenomenon highly common in the city of Bologna (2).
- Thermal insulation - The vegetative cover contributes to the reduction of thermal dispersion between indoor and outdoor with a reduction in energy consumption
- Acoustic insulation
- Bioclimatic improvement - The green shells, replenishing that lessened biological mass due to the cementation, allow through the photosynthesis the transformation of carbon dioxide into oxygen. In addition, by reducing the emissions caused by the use of air conditioning systems, they contribute to improving environmental viability.
- High water retention - the elements of the package and the substrates are able to return to the environment by evapotranspiration up to 80% of rainwater, thus reducing the flow to sewage
- Decrease in the heat island phenomenon - vegetation permits dust retention (7)

Implementation activities

At the end of the planning process, it took few months for the roof to be installed. The first months required more careful maintenance since it was over summer. Plants needed to be watered more frequently. Constant observation of the roof were carried both viasually and through specific sensors. The approach was keeping half of the roof of the university building as the conventional concrete roof and the other half as green roof, in order to make a comparison of their performances in terms of water retention and heat mitigation (1).

The project was partly financed by LIFE+. The LIFE programme is the EU’s funding instrument for the environment and climate action. The general objective of LIFE is to contribute to the implementation, updating and development of EU environmental and climate policy and legislation by co-financing projects with European added value (3).

... a national policy or strategy?Unknown

... a local policy or strategy?Yes

The intervention was part of the Environmental Sustainability Plan developed by the University of Bologna for the years 2013-2016 (3).

Monitoring

Expected impacts

Climate action for adaptation, resilience and mitigation (SDG 13)

Environmental quality, including air quality and waste management

Green space, habitats and biodiversity (SDG 15)

Regeneration, land-use and urban development

Water management (SDG 6)

Details on impacts and indicators used During rainfall events it is possible to study the behaviour of the impervious roof compared to the green roof. For example the rainfall event recorded on August 20th, 2013 (Fig. 3) is characterised by 2 rainfall peaks, the first with a maximum intensity of about 54 mm/hour, and, after one hour, the second peak of a maximum intensity of about 18 mm/hour and the total rainfall is about 9.7 mm. It is interesting to observe that the first peak generates a peak flow of about 0.24 L/s for the impervious roof and about 0.02 L/s for the green roof while the second rainfall peak generates a maximum flow of about 0.14 L/s for the impervious roof and about 0.05 L/s for the green roof, greater than the first peak due to the saturation of the soil. It is possible also to observe that the delay between the
rainfall peak and the maximum flow is about 2 minutes for the impervious roof and 13 minutes for the green roof (1 and 2).